Aluminium tube and threaded inserts for truss Dobsonian

[Stevo69]

I am ringing around for 1 1/4" aluminium tube for the trusses for the 18" Dobsonian project. Two parts to my question:


1. Threaded inserts: Does anyone know of a local supplier for the threaded inserts described in Kriege's book? If not, and I have to purchase from Obsession Telescopes (which I'm happy to do if necessary), will there be enough tolerance for 1.6mm tube.

2. Aliuminium Tube: All I can find so far is 1 1/4" (32mm) diameter, but 1.6mm thick. Now Kriege's book, suggests 0.049" (1.24mm) thickness walls. My two local suppliers are Smart Aluminium (http://www.smartaluminium.com.au/) and Williamstown Metal Merchants (http://www.metalstore.com.au/aboutUs.htm). I am looking for local suppliers so I can pickup, rather than delivery from interstate, and these two are on my side of Melbourne.

hoo roo,

Steve

[Rod]

Hi Steve,

I got some threaded inserts from
Hills Clearance Centre in Keysborough


Cambria Rd (cnr Bridge Rd) 3173

Ph: (03) 9238 2555

I am not sure if they have any as large as you want. I know they have 25mm but you may not need 1 1/4. Most people feel the Kriege design is overbuilt. Others with more engineering expertise could advise you there.

Hope that helps,

Rod.

[Stevo69]

Thanks Rod, I'll call them today. Yes, I have been told that the Kriege design is over-engineered. But I always err on the side of caution, but happy to go with 1" (25mm) tubes if enough ATM's convince me;)

[Satchmo]

Steve, You'll find a formula in Kreiges book, that gives a ratio of pole diameter to truss span of about 1/80 . So for under 2 meter length truss tube , 1" is fine. Wall thickness does little for stiffness but rapidly adds weight as you increase , which means larger mirror box to cope with higher side bearings, hence large rocker box etc., so I would use the 1.2mm wall.

Another advantage of using 1" tubes is that they will fit the new Moonlite aluminium split blocks, so using splits on the bottom and ball joints on the top makes for a very quick assembly both before and after you finish your scope.
I'm going to use this system on the 14" F4.75 binoculars I am building.
http://www.focuser.com/cgi-bin/dman.cgi?page=category&plugin=dstore.cgi&category=5

I'm also going to use their Delrin threaded tube inserts for the cage spacing tubes. I decided 20 X metal 1" threaded tube closures needed will add too much weight to my top end.

[Stevo69]

Thanks Mark, yeah I've decided to go with 1" tube. Unfortunately three companies I have contacted today in Melbourne only have 1" tube with 1.6mm walls. This mean the ID is 0.16mm smaller than a tube with 1.2mm ID. I think the tolerances on the Moonlite's Delrin threaded tube inserts will cater for this small amount, would you agree?

Also, I've looked again that the Moonlite truss parts. I would need 4 x TC100D (http://www.focuser.com/cgi-bin/dman.cgi?page=productdetail&plugin=dstore.cgi&product=TC-100D)for the secondary cage, and 8 x TC100S (http://www.focuser.com/cgi-bin/dman.cgi?page=productdetail&plugin=dstore.cgi&product=TC-100S)for the Mirror box.

Or would you use the 8 x TC100A (http://www.focuser.com/cgi-bin/dman.cgi?page=productdetail&plugin=dstore.cgi&product=TC-100A)for the mirror box?

And Mark, I as I may have mentioned to you over the phone last week, I have a habit of over-engineering things - a habit of mine which can get quite obsessive and unnecessary.

[Satchmo]

I 'd be using these (http://www.focuser.com/cgi-bin/dman.cgi?page=productdetail&plugin=dstore.cgi&product=TC-100A) for the bottom end as they hold all your poles at the right angle to drop on your cage. Use the twin ball/ socket pairs for the top. Too easy. :)

Heres a link showing a minimal 16" project (http://www.xmission.com/~alanne/DS4Main.html): its always good to see how people handle functional weight extremes.

Capral Aluminium in Sydney have 1" OD X 1.2mm wall , I'm not too familiar with Melbourne suppliers. You'd find that you'll need to machine the inserts for 1.6mm wall. You could also send Moonlite an off- cut as they'll machine for $1.50 each, though obtaining some 1.2mm would be the best outcome.

[GrahamL]

I got moonlite to machine for a 1.6 mm pole they charged $1 each
my scope 12" dosn't have very long poles though .

after the fact I noticed another rack with a thinner wall tube
it was marked as 25.4 mm 1.24 mm wall or something close
so it is around if you hunt for it.

good comany to deal with took about 10 days to do the custom order
3 days back to aus

cheers graham

[Stevo69]

How did you find the "fit" when when you actually put the machined parts into your 1.6mm-walled tube? Was it a good fit?

What keeps these parts in the tube? Are they just a very tight fit? Or does the action of screwing in a screw into the thread tighten it up?

Smart Aluminium (http://www.smartaluminium.com.au/) has 1" (1.6mm) in stock at $20.80 per 6.5m length and they will cut it for me.

[GrahamL]

The fits fine steve .. they make some pretty nice focusers from scratch so
a little fine tuneing of there tube inserts isn't a problem.. few differant ways of secureing them reading there advice here
http://www.focuser.com/cgi-bin/dman.cgi?page=trussconn


:)

[aerobrake]

Hi Steve,

I'm half way thru an 18in dob build as you know( using Kriege's book) and have on hand a set of moonlite twin ball/socket ends (for top and bottom) and have gone with 1 1/4in tubes. The fit is just a tad loose but a little bit of 5 minuite areldite will fix that problem. The instructions suggest putting a screw thru the tube into the derelin as well. Although I dont have these its a good suggestion to have split blocks on the bottom end so the poles are held upright, making your job of attaching the upper cage easy, otherwise its a two man job.

I got the threaded inserts from obsession and they are worth it.

Just to throw you a "wabbly" I did not consider the 1in tubes, I know Kriege's dobs are overbuilt but you are talking about a smaller diameter and thinner tube with the 1in comparred to the 1 1/4in tubes, too risky in my humble opinion.

When you build the upper cage and see the size of the bloody thing you will appreciate the larger tubes!!! Krieges book explains the mathametics of a smaller diameter which scarred me into the bigger tubes.

Adios
Mike

[CoombellKid]

I'm halfway through making my own wedge style cage connectors. They are
fairly easy to make if you have a router and a drill press.

regards,CS

[OneOfOne]

Have you tried Capral in Clayton? On Dandenong Rd, near Blackburn Rd. This is where I get all my aluminium. Unfortunately, they don't seem to have a catalogue of extrusions on their website, I was looking for some solid round bar around 120mm a couple of days ago...

[Satchmo]

*And yet the formulae he gives in the book suggests one inch tube for an 18" F4.5* . I think its really a case of people go for 1 1/4" tube to visually match the over built top end. For a 60" tube span on an F4.5 18" , 1" tube easily satisfies Kreiges 1/80 rule.

My attitude is to make every square mm of a telescope justify its existance. The primary enemy of large scopes is thermal mass , as you don't always have time to give your scope a long cooldown. Building your scope becomes a compromise on how heavy you are going to build vs the winds you intend to observe in.

[aerobrake]

G'day Mark, agreed with your comment but is that not with a slightly thicker walled alu tube? Or does the wall thickness not matter too much? I was thinking since the conversion from imperial to metric cant be matched exactly (ie the components available in OZ are usually a little undersized when trying to match whats in the dob book) would it not be wiser go go slightly bigger?

One thing I did not notice before is that Steves mirror is a F4.5, mine being a F5 so that will definitely make a difference.

Cheers
Mike

[Satchmo]

Hi Mike, If your pole length is shorter than 80" then 1" will do according to Kreige. I've used a 100" FL 18" F5.5 'scope with 1" tubes and couldn't fault it. I had a look at Kreige and he doesn't really discuss the wall thickness in relation to his formula.

Thickness is not so important as diameter, but if you were going to 1.6mm wall tube in 1" , you might as well use 32mm X 1.2 which would probably weigh similar. From memory the problem with long thin poles is resonant vibration in a breeze, though I've never seen it myself. He generally refers to cases of a classic 20" F5 scope. I certainly wouldn't use 1" poles on a 20" F5.

[Stevo69]

Hi Rod, I rang them today and described the picture of one in Kriege. Yes they do have them in stock, so I'll charge up the e-tag and drive to Keysborough next Monday.

Graham, I see you have used the double ball-joints on both ends. I'm almost convinced with the Moonlite connectors, but it will add up pretty pricey. 8 x USD$25.00 + ($16 for machining) = $200 plus. My second option is the Obsession truss tube clamps (http://obsessiontelescopes.com/ATM_parts/index.html) at USD$120. Actually I don't think they look too bad, and I'd have to make the mirror box clamps.

[GrahamL]

not much change from $300 actually.. I got a decent bonus from work so
it didn't bother me splashing a little .. well all of it on scope bits :)

[Stevo69]

Rob, they are fantastic! Top effort but I think I'd stuff them up.


Picked up the threaded inserts at Hills today. Bought a handfull of 1" ($1.20 each) and 1 1/4" ($1.00 each). Bargain!!

[CoombellKid]

Thanks Steve,

here's the finished product after staining them and before I sprayed them
with Estapol. They're presently being glued to the secondary cage.

regards,CS

[Rod]

Picked up the threaded inserts at Hills today. Bought a handfull of 1" ($1.20 each) and 1 1/4" ($1.00 each). Bargain!![/quote]

Glad it worked out Steve.

With the upper truss assembly I used the offset bracket design described by Krieg and Berry. They are cheap and easy to make. If you use their design, you have a loose wing nut or knob which is not much of a hassle - you can attach it with a short chain to the upper cage.

I am experimenting with keeping my struts in pairs, held together with a captive bolt. I haven't fully de-bugged it yet but happy to send a photo if you are interested.

Rod.

[Stevo69]

Rod, I'm intrigued! can you post any pictures to explain further. It sounds interesting!

[Rod]

Hi Steve,

The first photo shows the two joined trusses. On the side you can't see, I drilled a small hole through the bolt and placed a pin in it to hold everything together. The aluminium block is tapped to accept the bolt. The first few millimetres are drilled with a wider hole to accommodate the pin. It's a very crude attempt to do what is shown here:

http://www.wcc.net/~myastronomy/ATM/B3/Upper%20Cage_truss%20tubes.htm (http://www.wcc.net/%7Emyastronomy/ATM/B3/Upper%20Cage_truss%20tubes.htm)

As I have built it at this stage, collimation does not hold between setups so the idea needs some work. It is however very rigid.

Hope this gives you some ideas.

Rod.

[CoombellKid]

I thought about the loose wing nut in the wedge clamps I made, so what
I did to was to save the extra weight of a safety chain was smack the
thread ends with a flathead screwdriver, there is no way the wing nuts
can come free now without the use of a couple of pairs of pliers and or
damage to the clamps themselves. Besides I have to make it kid proof

just a thought

However wedge clamps are easy to make if you have a router and
a drill press, hell if I can make them anyone can. I mean the last time I
built something in wood was neary thirty years ago in high school.

regards,CS

[Stevo69]

Hi Rob, I checked out your web page and I see you successfully bent the 3-ply after soaking it in water overnight. Any issues in doing this? After it dried out, did it crack or split? I've never used this process to bend plywood in this fashion. I'm seriously thinking of doing this with my secondary cage.

[CoombellKid]

Hi Steve,

It was pretty easy doing a 734mm x 55mm strip. If I was doing a full size
cage I would think about making a mold and use veneer. You could make
it just as strong but lighter. I did try to use a clean bit with no voids but
one did sneak in. You can pretty much hold up in bright sunlight and see
them. I'm surprised it didn't snap to tell you the truth. It dried within about
5-6hours pretty much on a rather warm day. It pretty well kept it shape
or had no problem. No cracking just as you would of seen on that page
I wrap it around an old 8" newt tube and gave it a sand. Sticking it onto
the inside of the rings was a lil tricky. I used the inner circle from cutting
the rings and a bunch of lil wooden wedges I made and glued it. Tacking it
with tacks or nails seem like it might be a lil hairy as the ply was still fairly
rigid and there isn't much meat in 3mm ply.

regards,CS

[Rod]

Hi Steve,

If you want to use ply to line the secondary cage, I used 1.5 mm thick ply I bought from marine timbers in seaford. It was about $32 for a sheet of 8x4 gaboon. You don't have to do anything special to bend it and it is very strong.

Rod.

[Satchmo]

That would probably be a door skin ply ? In Sydney we have Mr Plywood which sells specialty plies including thin marine/aircraft ply. The 1.5mm is a great choice for cage lining. Kydex if it gets subjected to high heat in Summer as you might find in a locked car, will go dimply like a bad case of cellulite . I've seen a few Obsession style scopes suffering from the `fat thie' syndrome.

Rolled 1.5mm ply has a lot of inherent strength too. Monoquoque construction of rolled plywood panels is how they used to build aircraft. :)

[Stevo69]

Thanks Mark and Rod. I'm warming to the idea of a very thin plywood, such as this as a substitute for Kydex. Worst case scenario will be 3-ply soaked and bent, but I'll try to find a supplier for the 1.5mm plywood this week. I'm gathering all the secondary cage parts and materials for a monster working bee in 3 weekends.

[Rod]

Hi Mark,

I think it could be used for door skins. I bought it from a shop that supplies boat building materials so I assumed that it was some form of marine ply.

Rod

[Stevo69]

Yesterday I did some experimenting...

Experiment 1.

I bent some 3mm MDF in a 19" cylinder to prove it could be done. It is held in place nicely and there was no "creaking" or "splitting" as I was bending it. I'm now going to leave it for a week or so and see if it breaks. No soaking in water.

Experiment 2.

I bent another piece in the same way except I lightly sprayed it with a fine mist of water on on side before bending. Then I bent it into a cylinder, clamped it and immediately put it in a warm, well-heated room to dry. Two points I noticed: firstly, it bent easier when sprayed with a fine mist. Secondly, after a few hours, I removed the clamps and it retained more of its shape than the first experiment (no water).

I'm going to keep an eye on these two pieces and check their condition in a week, then I will seal them with primer, then semi-gloss black on the outside, and matt black on the inside. Then check them in another week.

A cylinder of 3mm MDF is very light in weight and I know it may be "heavy" and unacceptable to some, but I'm prepared to wear that.

Kydex is definately "out" in my books and a timber material is now a preference.

My only issue is that if I use this type of thickness, it will add approx 3-4mm to the ID of the secondary rings. Now, with Kriege's calculations, my secondary cage ID will be under size by approximately 1/8 - 3/16 smaller than the calculations. In the scheme of things, probably not noticeable.



Rod, You wouldn't have the name of the shop in Seaford? And the name of the material you purchased? I might also consider it for the secondary cage lining. Can you let me know?

hoo roo,
Steve

[Rod]

Hi Steve,

It is called gaboon and the shop addrss is:

Factory 3,
10 Rutherford Road, Seaford 3198 (facing Keppler Circuit)
Ph: 03 9775 0006 Fax: 03 9775 1776

Are you a member of the ASV and going to the meeting on Wednesday night? If so I can bring what I have left over for you. I'm not sure there is enough for your project. You would definitely have to lay it as two strips side by side. PM me if you are interested.

Rod.

[Stevo69]

Thanks Rod, sorry I didn't get back to you sooner, but I couldn't get to the Wednesday night meeting anyway. I'll give them a call first thing on Monday and might go for a day trip next Friday.